A radio connection over which radio communication can take place can be set up between a transmitter and a receiver in a radio communications system. The connection is two-directional, reference being made in this respect to a downlink which is the connection in a direction away from a base station in the radio communications system to a mobile station and an uplink which is the connection in the opposite direction, i.e. from the mobile station to the base station. The radio traffic on different connections is transmitted and received on radio channels that can be defined by a certain frequency in an FDMA system (Frequency Division Multiple Access), or by a combination of a certain frequency and a certain time slot in a system that uses TDMA (Time Division Multiple Access). In a CDMA system, (Code Division Multiple Access). A radio channel can be defined by a code.
The radio channels available in an FDMA system and a TDMA system can be reused. The signal strength received in a receiver on one radio channel may therefore include signal strength contributions from all transmitters that transmit on this radio channel. The distance between two transmitters that transmit on one and the same radio channel, the so-called repetition distance, should then be sufficiently large to avoid excessive co-channel interference on the desired received signal. In a CDMA system, the same frequencies can be used in all base stations in the radio communications system.
By interference is meant the sum of the signal strength of all received undesired signals on the radio channel used. These undesired signals arrive mainly from other connections that use the same radio channel in adjacent cells in the radio communications system. The received undesirable signals may also arrive from connections within the own cell, these connections using an adjacent frequency or time slot.
When the quality of a connection in a radio communications system is so poor as to be unacceptable, this may be due to the ratio between the signal strength of the desired signal and the interference being too low, among other things.
The signal strength or power level received by a receiver, e.g. a base station receiver, on a certain radio channel may thus contain contributions from several different transmitters in mobile stations that transmit on this radio channel in the radio communications system. The signal strength ratio between the desired signal (carrier) and the interfering signals is normally referred to as the C/I ratio (Carrier to Interference ratio) and is a measurement of call or channel quality.
The strength of the received signals will depend on the transmission power levels of those transmitters that transmit on the radio channel in question and to the extent to which the signals are attenuated on their way from the transmitters to the receiver. Attenuation, or path loss, is contingent on the distance, the direction and the topology between transmitter and receiver, among other things. Path loss can be seen as the inverse of radio channel amplification between values of 0 and 1, i.e. reduces along its path from the transmitter to the receiver. Thus, if a signal is attenuated five times, its amplification will be 0.2. Amplification can also be expressed in decibels, these values then becoming negative. Path gain and path loss are terms that can be used parallel with attenuation, as those skilled in this art are well aware.
The signal strength on a given radio channel can be measured at a receiver and the measured value then used as an indication of radio channel interference.
U.S. Patent Specification 5,157,709 describes an adaptive radio communications system that includes a control station which sets up an interference matrix for base interference values between base stations. Each base station measures power levels of signals received on idle radio channels. Information relating to the power levels received on idle radio channels and to the identity of the radio channels and base station are forwarded to the control station. The control station generates, on the basis of this information, an interference matrix that is used in adaptive channel allocation to the base stations.
U.S. Patent Specification 5,287,544 describes a method of obtaining a certain C/I value in a radio communications system with reduced transmitter power level. Idle radio channels are divided into groups in accordance with similar interference properties. Signal attenuation on the downlink between a base station and a mobile station is measured and the corresponding uplink value is calculated. Channels are allocated by selecting a radio channel from a channel group such as to obtain a certain desired C/I value at a certain transmission power level.
EP Patent Specification 0673129 describes a method of determining directly the C/I value in respect of co-channel and adjacent-channel interference. Received signals are processed mathematically in several stages, so as to finally give the C/I values.
For instance, the magnitude of the interference that a radio channel that shall be allocated to a certain connection will obtain can be predicted by estimating the interference or determining the C/I level in a radio communications system. One drawback with such methods, however, is that it is not possible to foresee how this newly established connection will influence connections that have already been set up and that use the same radio channel in the system.